A Little Math Lesson: Are Used Cars Greener Than Hybrids?

It's a common question: Which is greener, continuing to drive my old car or buying a new, more fuel-efficient one?

As with many such questions, the answer is: It depends.

But one of the things it depends on is doing the math right. As much as we love CBS's The Fast Draw segments, we think Mitch Butler and Josh Landis got it wrong a few years ago when they compared a new hybrid to a used car.

Watch the segment above, then we'll discuss.

Done? OK, here we go.

It's true that the energy used for materials and manufacturing for a car has a noticeable carbon footprint. The Quick Draw folks cite "an engineer" who "figured out" that manufacturing the Prius is the equivalent of 1,000 gallons of gasoline.

We think that's high (see below), but let's use their figure.

So if you were able to use that used car for just as many miles as you'd be able to use that new Prius, then theoretically the Prius wouldn't need to be built. That would cut carbon emissions.

But by how much? Let's look at the math. Over a 10-year life at 15,000 miles per year, a brand-new, 50-mpg 2012 Toyota Prius will use 3,000 gallons of gasoline.

If your hypothetical used car gets 33 mpg (to make the math easy) and lasts long enough after you buy it to cover that same 150,000 miles, you'll use half again as much gasoline, or 4,500 gallons.

That means that the Prius still saves 500 gallons of gasoline over its lifetime (if you accept the 1,000-gallon manufacturing equivalent).

Ford's 100,000th hybrid rolls off the Kansas City assembly line in March 2009

There are two flaws in that argument, though. First, you probably won't be able to use that used car as long as you will a brand-new hybrid.

All cars have a lifecycle, and somewhere around 10 to 12 years and 100,000 to 150,000 miles, most cars start failing in increasingly expensive ways. A 3-year-old used car may cover almost as many miles as a new one--but on average, it won't.

Second, and more important, every car has a lifecycle carbon impact that has to be apportioned to the mileage it covers over its total life. You can't allocate all of that energy to the owner who drove it only for a couple of years before you bought it.

So you have to include the materials and manufacturing energy for your used car, too, over the fraction of its life you own it. That number may be lower than 1,000 gallons, but it's not zero. The Fast Draw folks don't say what it is--but they should have.

And according to a study from the Massachusetts Institute of Technology, lifetime carbon is far lower overall for the higher-efficiency car even if the portion from manufacturing is higher.

So that study may indicate the Fast Draw estimate is high. Even in the case of an average 25-mpg car, MIT says materials and manufacturing may only total 360 gallons--which is why the 1,000 gallons cited on the segment for the Prius sounds high.

Hybrids may require more raw materials and produce a higher carbon footprint due to their battery pack and electric machinery, but we're skeptical that it's three times as high. Still, since the segment doesn't identify the study, we're at a standstill.

Conclusion: Over the lifetime of the vehicle, it's all about the fuel the car uses. And even if you accept the entire 1,000 gallons without accounting for the used car at all, the hybrid's fuel savings still make it come out ahead.